L 'Ligands on the Structure

It has been suggested that the reactivity of π-donating ligands bound to late-transition-metal complexes is heightened due to high d-electron counts...
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Inorg. Chem. 2002, 41, 3042−3049

Influence of Filled dπ-Manifold and L/L′ Ligands on the Structure, Basicity, and Bond Rotations of the Octahedral and d6 Amido Complexes TpRu(L)(L′)(NHPh) (Tp ) Hydridotris(pyrazolyl)borate; L ) L′ ) PMe3 or P(OMe)3, or L ) CO and L′ ) PPh3): Solid-State Structures of [TpRu(PMe3)2(NH2Ph)][OTf], [TpRu{P(OMe)3}2(NH2Ph)][OTf], and TpRu{P(OMe)3}2(NHPh) David Conner, K. N. Jayaprakash, T. Brent Gunnoe,* and Paul D. Boyle Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204 Received February 28, 2002

It has been suggested that the reactivity of π-donating ligands bound to late-transition-metal complexes is heightened due to high d-electron counts. Herein, the synthesis and characterization of the Ru(II) amine and Ru(II) amido complexes [TpRuL2(NH2Ph)][OTf] (OTf ) trifluoromethanesulfonate) and TpRuL2(NHPh) (L ) PMe3 or P(OMe)3) are presented, including solid-state X-ray diffraction studies of [TpRu(PMe3)2(NH2Ph)][OTf], [TpRu{P(OMe)3}2(NH2Ph)][OTf], and TpRu{P(OMe)3}2(NHPh). The pKa’s of the Ru(II) amine complexes and the previously reported [TpRu(CO)(PPh3)(NH2Ph)]+ have been estimated to be comparable to that of malononitrile in methylene chloride. In addition, the impact of the filled dπ-manifold (i.e., Ru(II) and d6 octahedral systems) on barriers to rotation of the Ru−NHPh moieties has been studied. For TpRu(PMe3)2(NHPh) and TpRu{P(OMe)3}2(NHPh), evidence for hindered rotation about the amido nitrogen and phenyl ipso carbon has been observed, and the relative N−C and Ru−N bond rotational barriers for the series of three amido complexes are discussed in terms of the π-conflict.

Introduction Late-transition-metal amido complexes are known to be important to a variety of important reactions.1-4 Since the homolytic bond strengths of late transition metal amido bonds have been demonstrated to be relatively strong,5,6 alternative explanations for the scarcity and high reactivity of such complexes have been proposed. For example, it has been suggested that the π-conflict between filled metal dπ-orbitals and ligand-based electron pairs is responsible, at least in part, for the paucity of late-transition-metal complexes with π-donating ligands.7,8 Along these lines, examples of octa-

hedral and d6 amido complexes are relatively rare, and this is especially true for systems that lack aryl substituents on the amido ligand.5,6,9-21 Efforts directed toward the synthesis and reactivity of late transition metal amido complexes and

* Author to whom correspondence should be addressed. Fax: (919) 5158909. E-mail: [email protected]. (1) Hartwig, J. F. Angew. Chem., Int. Ed. 1998, 37, 2046-2067. (2) Roundhill, D. M. Catal. Today 1997, 37, 155-165. (3) Roundhill, D. M. Chem. ReV. 1992, 92, 1-27. (4) Mu¨ller, T. E.; Beller, M. Chem. ReV. 1998, 98, 675-703. (5) Bryndza, H. E.; Fong, L. K.; Paciello, R. A.; Tam, W.; Bercaw, J. E. J. Am. Chem. Soc. 1987, 109, 1444-1456. (6) Hartwig, J. F.; Andersen, R. A.; Bergman, R. G. Organometallics 1991, 10, 1875-1887. (7) Caulton, K. G. New J. Chem. 1994, 18, 25-41. (8) Mayer, J. M. Comments Inorg. Chem. 1988, 8, 125-135.

(9) Kaplan, A. W.; Ritter, J. C. M.; Bergman, R. G. J. Am. Chem. Soc. 1998, 120, 6828-6829. (10) Fulton, J. R.; Bouwkamp, M. W.; Bergman, R. G. J. Am. Chem. Soc. 2000, 122, 8799-8800. (11) Hartwig, J. F.; Andersen, R. A.; Bergman, R. G. J. Am. Chem. Soc. 1989, 111, 2717-2719. (12) Macgregor, S. A.; MacQueen, D. Inorg. Chem. 1999, 38, 4868-4876. (13) Woerpel, K. A.; Bergman, R. G. J. Am. Chem. Soc. 1993, 115, 78887889. (14) Hsu, G. C.; Kosar, W. P.; Jones, W. D. Organometallics 1994, 13, 385-396. (15) Glueck, D. S.; Bergman, R. G. Organometallics 1991, 10, 14791486. (16) Glueck, D. S.; Winslow, L. J. N.; Bergman, R. G. Organometallics 1991, 10, 1462-1479. (17) Flood, T. C.; Lim, J. K.; Deming, M. A.; Keung, W. Organometallics 2000, 19, 1166-1174. (18) Dewey, M. A.; Arif, A. M.; Gladysz, J. A. J. Chem. Soc., Chem. Commun. 1991, 712-714. (19) Dewey, M. A.; Knight, D. A.; Arif, A.; Gladysz, J. A. Chem. Ber. 1992, 125, 815-824. (20) Boncella, J. M.; Eve, T. M.; Rickman, B.; Abboud, K. A. Polyhedron 1998, 17, 725-736.

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© 2002 American Chemical Society Published on Web 04/27/2002

Influence of the Filled dπ-Manifold and Ligands Scheme 1. The Presence of π-Conflict Leads to a Filled Antibond π*-Molecular Orbital

related systems have begun to delineate the reactivity patterns of such complexes.22,23 According to the π-conflict theory, the reactive nature of high d-electron count complexes with π-donating ligands is derived from an occupied antibonding π*-orbital (Scheme 1); however, the extent to which the π-conflict contributes to the reactivity of late transition metal amido complexes and the factors that control the π-conflict are not well understood.24 Given the presence of late transition metal amido bonds in important reactions, understanding the features that modulate amido reactivity (and possibly the π-conflict) is important. Here, we report studies of TpRu(L)(L′)(NHPh) in which L ) L′ ) PMe3 or P(OMe)3, or L ) CO and L′ ) PPh3, including the basicity of the amido ligands and the relative Ru-Namido and Namido-Cphenyl bond rotational barriers. To our knowledge, the TpRu(L)(L′)(NHR) (R ) Ph or H) systems are the first examples of a series of octahedral and d6 amido complexes in which the ancillary ligands are varied.25 Experimental Section General Methods. All reactions and procedures were performed under anaerobic conditions in a nitrogen-filled glovebox or by using standard Schlenk techniques. Glovebox purity was maintained by periodic nitrogen purges and monitored by an oxygen analyzer (O2(g) < 15 ppm, for all reactions). Hexanes were purified by passage through a column of activated alumina.26 THF, toluene, benzene, and diethyl ether were dried by distillation over sodium/benzophenone. Benzene-d6 and CD3CN were purified by distillation from CaH2, degassed, and stored over 4 Å sieves. CDCl3 and CD2Cl2 were degassed via three freeze-pump-thaw cycles and stored over 4 Å sieves. 1H and 13C NMR spectra were obtained on a Varian Mercury 300 MHz or General Electric 300 MHz spectrometer. Resonances due to the Tp ligand are reported by chemical shift and multiplicity only. All 3JHH values for the pyrazolyl rings are 2 Hz. All 1H and 13C NMR spectra were referenced against tetramethylsilane using residual proton signals (1H NMR) or the 13C resonances of the deuterated solvent (13C NMR). 31P NMR spectra (21) Joslin, F. L.; Johnson, M. P.; Mague, J. T.; Roundhill, D. M. Organometallics 1991, 10, 2781-2794. (22) Bryndza, H. E.; Tam, W. Chem. ReV. 1988, 88, 1163-1188. (23) Fulton, J. R.; Holland, A. W.; Fox, D. J.; Bergman, R. G. Acc. Chem. Res. 2002, 35, 44-56. (24) Holland, P. L.; Andersen, R. A.; Bergman, R. G. Comments Inorg. Chem. 1999, 21, 115-129. (25) Jayaprakash, K. N.; Conner, D.; Gunnoe, T. B. Organometallics 2001, 20, 5254-5256. (26) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518-1520.

were obtained on a Varian 300 MHz spectrometer and referenced against external 85% H3PO4. All variable-temperature NMR experiments were performed on a General Electric 300 MHz spectrometer. UV-vis spectra were obtained on a Varian Cary 3E spectrophotometer. IR spectra were obtained on a Mattson Genesis II spectrometer either as thin films on a KBr plate or in a solution using a KBr solution plate. Electrochemical experiments were performed under a nitrogen atmosphere using a BAS Epsilon Potentiostat. Cyclic voltammograms were recorded in a standard three-electrode cell from -2.00 to +2.00 V with a glassy carbon working electrode and tetrabutylammonium hexafluorophosphate as an electrolyte. Tetrabutylammonium hexafluorophosphate was dried under a dynamic vacuum at 110 °C for 48 h prior to use. All potentials are reported versus NHE (normal hydrogen electrode) using cobaltocenium hexafluorophosphate as an internal standard. Elemental analyses were performed by Atlantic Microlabs, Inc. The syntheses of TpRu(CO)(PPh3)(NHPh) (1), TpRu(PMe3)2(OTf) (2), TpRu{P(OMe)3}2(Cl) and TpRu{P(OMe)3}2(OTf) (3) have been previously reported.25,27 Li{(NC)2CH} was generated by the reaction of malononitrile with 1 equiv of n-BuLi in benzene followed by vacuum filtration to collect the resulting white precipitate. All other reagents were used as purchased from commercial sources. [TpRu(PMe3)2(NH2Ph)][OTf] (4). TpRu(PMe3)2(OTf) (2) (1.3674 g, 2.222 mmol) was dissolved in approximately 50 mL of THF (OTf ) trifluoromethanesulfonate). To this solution was added aniline (2.1295 g, 21.6 mmol), and the resulting mixture was allowed to stir for 24 h. The solution was concentrated to approximately 20 mL in vacuo, and diethyl ether (approximately 40 mL) was added to precipitate the product. The product was collected via vacuum filtration through a fine porosity frit and washed with diethyl ether (3 × 10 mL) to give a white solid (1.4089 g, 1.988 mmol, 90%). IR (thin film on KBr plate): νNH ) 3285 and 3262 cm-1, νBH ) 2481 cm-1. UV-vis (THF): λmax ) 267 nm ( ) 0.96 × 104 M-1 cm-1). 1H NMR (CD3CN, δ): 7.85, 7.25 (each 2H, each a d, Tp CH 3 or 5 position), 7.80, 7.38 (each 1H, each a d, Tp CH 3 or 5 position), 6.95 (3H, m, overlap of NH2C6H5 meta and para), 6.35 (2H, d, 3JHH ) 7 Hz, NH2C6H5 ortho), 6.18 (2H, t, Tp CH 4 position), 6.14 (1H, t, Tp CH 4 position), 4.77 (2H, br s, NH2Ph), 1.30 (18H, vt, JPH ) 7 Hz, P(CH3)3). 13C{1H}NMR (CD3CN, δ): 145.8, 142.7, 142.5, 136.9, 136.3 (each an s, Tp CH 3 or 5 position, and amine phenyl ipso carbon), 128.8, 125.1, 122.7 (each an s, amine phenyl ortho, meta, and para), 106.7, 106.0 (each an s, Tp CH 4 position), 18.1 (d, 1JPC ) 12 Hz). 31P{1H}NMR (CDCl3, δ): 10.9. CV (THF, TBAH, 100 mV/s): Ep,a ) 1.82 V (Ru(III/II)). Anal. Calcd for C22H35BF3N7O3P2RuS: C, 37.29; H, 4.98; N, 13.84. Found: C, 37.16; H, 4.95; N, 13.61. [TpRu{P(OMe)3}2(NH2Ph)][OTf] (5). To a solution of TpRu{P(OMe)3}2Cl (1.2587 g, 2.106 mmol) in approximately 50 mL of THF was added AgOTf (0.5413 g, 2.107 mmol). The resulting solution was refluxed for 20 h. During the reaction, the formation of a white precipitate (AgCl) was noted. The solution was cooled to room temperature and filtered through a fine porosity frit. Aniline (2.0564 g, 20.9 mmol) was added to the resulting solution. The mixture was allowed to stir for an additional 24 h. The solution was concentrated to approximately 20 mL in vacuo, and diethyl ether (approximately 40 mL) was added to precipitate the product. The product was collected via vacuum filtration through a fine porosity frit and washed with diethyl ether (3 × 10 mL) to give a white solid (1.5823 g, 1.967 mmol, 93%). IR (thin film on KBr plate): νNH ) 3318 and 3250 cm-1, νBH ) 2450 cm-1. UV-vis (THF): λmax ) 223 nm ( ) 2.0 × 104 M-1 cm-1). 1H NMR (CD2(27) Jayaprakash, K. N.; Gunnoe, T. B.; Boyle, P. B. Inorg. Chem. 2001, 40, 6481-6486.

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Conner et al. Cl2, δ): 7.77, 7.39 (each 2H, each a d, Tp CH 3 or 5 position), 7.71, 7.63 (each 1H, each a d, Tp CH 3 or 5 position), 6.99 (3H, m, overlapping NH2C6H5 meta and para), 6.35 (2H, d, 3JHH ) 7 Hz, NH2C6H5 ortho), 6.16 (3H, m, overlapping Tp CH 4 position), 4.74 (2H, br s, NH2C6H5), 3.44 (18H, vt, JPH ) 10 Hz, P(OCH3)3). 13C{1H} NMR (CD Cl , δ): 148.3, 144.7, 137.7, 137.0 (each an s, 2 2 Tp CH 3 or 5 position), 142.7 (amine phenyl ipso carbon), 129.9, 127.1, 124.7 (each an s, amine phenyl ortho, meta, and para), 106.8, 106.7 (each an s, Tp CH 4 position), 53.2 (br s, P(OCH3)). 31P{1H} NMR (CD3CN, δ): 139.3 (s, P(OCH3)3). CV (THF, TBAH, 100 mV/s): Ep,a ) 1.86 V (Ru(III/II)). Anal. Calcd for C22H35BF3N7O9P2RuS: C, 32.85; H, 4.39; N, 12.19. Found: C, 33.02; H, 4.28; N, 12.24. TpRu(PMe3)2(NHPh) (6). A colorless solution of [TpRu(PMe3)2NH2Ph][OTf] (4) (0.1151 g, 0.162 mmol) in approximately 20 mL of THF was cooled to -78 °C. Sodium bis(trimethylsilyl)amide (0.178 mmol, 1.0 M in THF) was added dropwise via a microsyringe. After the addition of base, the color of the solution was noted to be pale yellow. The solution was allowed to warm to room temperature, and a color change to dark brown/yellow was noted. The volatiles were removed under reduced pressure. The resulting solid was dissolved in benzene (approximately 60 mL) and filtered through a fine porosity frit. Benzene was removed from the filtrate under reduced pressure to yield a pale yellow solid (0.082 g, 0.147 mmol, 90%). IR (solution cell in THF): νNH ) 3326 cm-1, νBH ) 2467 cm-1. UV-vis (THF): λmax ) 293 nm ( ) 1.7 × 104 M-1 cm-1). 1H NMR (C6D6, δ): 7.66, 7,47 (each 2H, each a d, Tp CH 3 or 5 position), 7.51, 6.89 (each 1H, each a d, Tp CH 3 or 5 position), 7.22 (2H, t, 3JHH ) 7 Hz, phenyl meta), 6.45 (1H, t, 3JHH ) 7 Hz, phenyl para), 5.88 (2H, t, Tp CH 4 position), 5.85 (1H, t, Tp CH 4 position), 2.80 (1H, br s, NHPh), 0.92 (18H, vt, JPH ) 6 Hz, P(CH3)3). At room temperature in C6D6, the resonances due to the ortho protons are broadened into the baseline. In CD2Cl2 at room temperature, the ortho protons are observed as a broad singlet at approximately 5.8 ppm. 13C{1H} NMR (C6D6, δ): 163.3 (s, Ph ipso), 145.7, 142.8, 136.0, 135.8 (each an s, Tp CH 3 or 5 position), 129.4, 128.5, 115.9 (each an s, amido phenyl ortho, meta, and para), 106.0, 105.7 (each an s, Tp CH 4 position), 18.9 (d, 1JPC ) 12 Hz, P(CH3)3). 31P{1H} NMR (C6D6, δ): 17.5 (s, P(CH3)3). CV (THF, TBAH, 50 mV/s): E1/2 ) -0.28 V (Ru(III/II)). Anal. Calcd for C21H31BN7P2Ru: C, 45.17; H, 6.14; N, 17.56. Found: C, 45.29; H, 6.17; N, 17.29. TpRu{P(OMe)3}2(NHPh) (7). A colorless solution of [TpRu{P(OMe)3}2NH2Ph][OTf] (5) (0.1142 g, 0.142 mmol) in approximately 20 mL of THF was cooled to -78 °C. To this solution was added dropwise via a microsyringe sodium bis(trimethylsilyl)amide (0.156 mmol, 1.0 M in THF) . After the addition of amide, the color of the solution was noted to be yellow. The solution was allowed to warm to room temperature, and a color change to dark yellow was noted. Removal of the volatiles under reduced pressure yielded a yellow solid. The resulting solid was dissolved in benzene (approximately 60 mL) and filtered through a fine porosity frit. Benzene was removed from the filtrate under reduced pressure to yield a bright yellow solid (0.085 g, 0.130 mmol, 91%). IR (solution cell in THF): νNH ) 3378 cm-1, νBH ) 2467 cm-1. UV-vis (THF): λmax ) 284 nm ( ) 1.3 × 104 M-1 cm-1). 1H NMR (C6D6, δ): 7.98, 7.56 (each 2H, each a d, Tp CH 3 or 5 position), 7.87, 7.46 (each 1H, each a d, Tp CH 3 or 5 position), 6.93 (2H, t, 3JHH ) 7 Hz, phenyl meta), 6.36 (1H, t, 3JHH ) 7 Hz, phenyl para), 6.00 (3H, m, overlap of Tp CH 4 position and phenyl ortho), 5.86 (1H, t, Tp CH 4 position), 3.14 (18H, vt, JPH ) 10 Hz, P(OCH3)3), 1.98 (1H, br s, NHPh). 13C{1H} NMR (C6D6, δ): 163.3 (s, phenyl ipso), 145.7, 142.8, 136.0, 135.8 (each an s, Tp CH 3 or 5 position),

3044 Inorganic Chemistry, Vol. 41, No. 11, 2002

129.4, 128.5, 115.9 (each an s, amido phenyl ortho, meta, and para), 106.0, 105.7 (each an s, Tp CH 4 position), 18.9 (d, 1JPC ) 12 Hz, P(OCH3)3). 31P{1H} NMR (THF-d8, δ): 137.5 (s, P(CH3)3). CV (THF, TBAH, 50 mV/s): E1/2 ) -0.25 V (Ru(III/II)). Anal. Calcd for C21H34BN7P2O6Ru-C6H6: C, 44.27; H, 5.50; N, 13.38. Found: C, 42.50; H, 5.42; N, 13.22. NMR Tube Reactions of Amido Complex 1, 6, or 7 with Malononitrile. All reactions followed the same general procedure. Inside a nitrogen-filled glovebox, an NMR tube was charged with a known amount of the appropriate amido complex. One equivalent of malononitrile was weighed out and dissolved in CD2Cl2, and this solution was added to the NMR tube with the amido complex. A 1H NMR spectrum was immediately acquired at room temperature. Variable-temperature 1H NMR experiments were performed by lowering the temperature of the NMR probe to -90 °C. UV-Vis Experiments for Reactions of Amido Complex 6 or 7 with Malononitrile. The procedure that was utilized for the 1HNMR spectra of complexes 6 and 7 with malononitrile was followed. The reaction solutions were transferred to a quartz cuvette (sealed under nitrogen), and UV-vis spectra were acquired. Crystal Structures of [TpRu(PMe3)2NH2Ph][OTf] (4), [TpRu{P(OMe)3}2NH2Ph][OTf] (5), and TpRu{P(OMe)3}2(NHPh) (7). For full details of X-ray structure determination of these complexes, please see Supporting Information.

Results We have previously reported the synthesis of TpRu(CO)(PPh3)(NHPh) (1) as well as TpRu(PMe3)2(OTf) (2) and TpRu{P(OMe)3}2(OTf) (3) (OTf ) trifluoromethanesulfonate).25,27 Ligand exchange of triflate upon reaction of 2 or 3 with excess aniline yields the corresponding Ru(II) amine complexes (eq 1). The amine complexes are charac-

terized by broad resonances for the amine protons at 4.77 and 4.74 ppm in the 1H NMR spectra of 4 and 5, respectively, and cyclic voltammetry experiments reveal irreversible oxidation waves at 1.82 and 1.86 V (versus NHE). The amine ligands of 4 and 5 are deprotonated upon treatment with NaN(SiMe3)2 to yield the amido complexes 6 and 7 (eq 2).

The amido protons for 6 and 7 resonate at 2.80 and 1.98 ppm, respectively, in their 1H NMR spectra. The negative shifts for the Ru(III/II) oxidation potentials relative to the

Influence of the Filled dπ-Manifold and Ligands Table 2. Selected Bond Distances (Å) and Angles (deg) for [TpRu(PMe3)2(NH2Ph)][OTf)] (4), [TpRu{P(OMe)3}2(NH2Ph)][OTf] (5), and TpRu{P(OMe)3}2(NHPh) (7) Bond Distances atoms

complex 4

complex 5

complex 7

Ru-P1 Ru-P2 Ru-N1 Ru-N3 Ru-N5 Ru-N7 N7-C10

2.3064(9) 2.3116(10) 2.091(3) 2.176(3) 2.158(3) 2.211(3) 1.444(5)

2.2336(6) 2.2209(6) 2.0947(18) 2.1620(18) 2.1620(18) 2.1822(19) 1.451(3)

2.2275(5) 2.2186(5) 2.1111(15) 2.1772(15) 2.1532(15) 2.1012(15) 1.365(2)

Bond Angles

Figure 1. ORTEP diagram of [TpRu(PMe3)2(NH2Ph)][OTf] (4). Table 1. UV-Vis Data for Ru(II) Amine and Ru(II) Amido Complexes in THF complex

λmax (nm)

 (M-1 cm-1)

[TpRu(CO)(PPh3)(NH2Ph)][OTf] TpRu(CO)(PPh3)(NHPh) (1) [TpRu(PMe3)2(NH2Ph)][OTf] (4) [TpRu{P(OMe)3}2(NH2Ph)][OTf] (5) TpRu(PMe3)2(NHPh) (6) TpRu{P(OMe)3}2(NHPh) (7)

207 216 267 223 293 284

8.2 × 104 1.2 × 104 0.96 × 104 2.0 × 104 1.7 × 104 1.3 × 104

oxidation potentials of the amine complexes 4 and 5 (E1/2 ) -0.28 and -0.25 V for 6 and 7, respectively) are consistent with the deprotonation reactions. UV-vis data for the series of amine and amido complexes [TpRu(L)(L′)(NH2Ph)][OTf] and TpRu(L)(L′)(NHPh) are shown in Table 1. The X-ray structure of [TpRu(PMe3)2(NH2Ph)][OTf] (4) shows a pseudooctahedral coordination sphere (Figure 1). Data collection parameters and selected bond distances and angles are displayed in Tables 2 and 3. The Ru-P bond distances are 2.3064(9) and 2.3116(10) Å. The Ru-N7C10 bond angle is 123.6(2)°, and the N7-C10 bond distance is 1.444(5) Å. The N-C bond distance of aniline is 1.398 Å.28 The Ru-N7 bond distance (2.211(3) Å) is slightly longer than the Ru-N bond distances of the Tp ligand (average Ru-N bond distance for the three pyrazolyl rings is 2.142(5) Å). The shorter Ru-N1 (Tp) bond distance (2.091(5) Å) compared with the other two Ru-pyrazolyl bond distances (average 2.167(4) Å) is indicative of an amine trans effect. Evidence of hydrogen bonding between the aniline ligand and triflate anion is observed in the structure of 4. For example, bond distances between the amine protons and triflate oxygen atoms are 2.20(6) Å (H7(a)-O1) and 2.06(4) Å (H7(b)-O2), with N7-O1 and N7-O2 distances of 2.966(4) and 3.018(4) Å, respectively. Inspection of the ORTEP diagram for [TpRu{P(OMe)3}2NH2Ph][OTf] (5) indicates an amine orientation that is analogous to that of complex 4 (Figure 2). The Ru-phosphite bond distances of complex 5 are 2.2336(6) and 2.2209(6) Å. The Ru-N7-C10 bond angle (120.78(14)°) for the (28) Fukuyo, M.; Hirotsu, K.; Higuchi, T. Acta Crystallogr., Sect. B 1982, 38, 640-643.

atoms

complex 4

complex 5

complex 7

P1-Ru-P2 Ru1-N7-C10 N7-C10-C11 N7-C10-C15 N1-Ru-N3 N1-Ru-N5 N3-Ru-N5 N3-Ru-N7 P1-Ru-N7 P2-Ru-N7 N1-Ru-N7

100.79(3) 123.6(2) 120.5(3) 119.6(3) 86.77(11) 88.65(11) 83.02(11) 90.51(11) 88.71(8) 95.79(9) 176.74(11)

91.29(2) 120.78(14) 119.4(2) 120.3(2) 87.57(7) 85.74(7) 83.96(7) 87.90(7) 86.81(5) 92.45(6) 175.43(7)

92.822(17) 135.30(13) 120.50(16) 115.84(16) 85.71(6) 86.24(6) 83.98(6) 92.45(6) 90.01(4) 89.52(4) 176.89(6)

Table 3. Selected Crystallographic Data and Collection Parameters for [TpRu(PMe3)2(NH2Ph)][OTf] (4), [TpRu{P(OMe)3}2(NH2Ph)][OTf] (5), and TpRu{P(OMe)3}2(NHPh) (7)

formula mol wt cryst syst space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z Dcalcd, g cm-3 total no. of reflns unique reflns R Rw

complex 4

complex 5

complex 7

C22H34BF3N7O3P2RuS 707.43 triclinic P1h 9.4185(10) 11.9282(14) 14.0374(10) 91.844(13) 93.045(13) 106.552(8) 1507.7(3) 2 1.558 5244

C22H35BF3N7O9P2RuS 804.43 monoclinic P21/c 9.1753(6) 23.7851(18) 15.3015(10)

3336.6(4) 4 1.601 5817

C27H40BN7O6P2Ru 732.48 triclinic P1h 9.8449(4) 11.2687(4) 14.8435(5) 86.667(3) 87.380(4) 87.854(4) 1641.22(10) 2 1.482 5751

5244

5817

5397

0.028 0.045

0.025 0.030

0.022 0.038

92.307(5)

phosphite complex is slightly reduced compared with that of complex 4, and the N7-C10 bond distance (1.4531(3) Å) is close to that of complex 4. The Ru-N7 bond distance (2.1822(19) Å) of complex 5 is shorter than that of complex 4 and is only slightly longer than the Ru-pyrazolyl bond distances (the average Ru-pyrazolyl bond distance is 2.139(31) Å). As with complex 4, the Ru-pyrazolyl-nitrogen bond distance trans to the amine ligand of complex 5 (2.0947(18) Å) is longer than the other two Ru-pyrazolyl bond distances (average bond distance of 2.1620(25) Å). Similar to complex 4, hydrogen bonding is observed between the amine ligand and triflate anion (see Supporting Information for details). The Ru-P bond distances (2.2275(5) and 2.2186(5) Å) of TpRu{P(OMe)3}2(NHPh) (7) are only slightly shorter than those of complex 5 (Figure 3). The Ru-N7 bond distance Inorganic Chemistry, Vol. 41, No. 11, 2002

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Conner et al.

Figure 3. ORTEP diagram of TpRu{P(OMe)3}2(NHPh) (7). Figure 2. ORTEP diagram of [TpRu{P(OMe)3}2(NH2Ph)][OTf] (5).

of 2.1012(15) Å is almost identical to the Ru-N1 bond distance (2.1111(15) Å) of the trans-pyrazolyl ring. The average of the Ru-pyrazolyl bond distances is 2.1472(25) Å, and the Ru-N7-C10 bond angle is 135.30(13)°. The N7-C10 bond distance is 1.365(2) Å and is shorter than the analogous distance for the Ru(II) amine complexes 4 and 5. Recently, Bergman et al. have reported that the parent amido ligand of trans-(DMPE)2Ru(H)(NH2) (DMPE ) 1,2bis(dimethylphosphinoethane)) is highly basic,9,10 and we have reported similar observations for a series of TpRu(L)(L′)(NH2) complexes.25 Given the involvement of late transition metal amido complexes in catalytic reactions, the impact of a filled dπ-manifold on the reactivity of the amido ligands is of interest. The synthesis of the amine complexes [TpRu(PMe3)2(NH2Ph)][OTf] (4) and [TpRu{P(OMe)3}2(NH2Ph)][OTf] (5) in the presence of excess aniline indicates that the free amine is not basic enough to deprotonate the metal-bound amine (eq 3).

This is in contrast to the octahedral and d4 tungsten complexes [Tp*W(CO)(PhC2H)(NH2Ph)]+ and [Tp*W(CO)2(NH2Ph)]+ (Tp* ) hydridotris(3,5-dimethylpyrazolyl)borate) that are deprotonated to form amido complexes in the presence of excess aniline.29,30 The pKa of PhNH3+ is (29) Powell, K. R.; Pe´rez, P. J.; Luan, L.; Feng, S. G.; White, P. S.; Brookhart, M.; Templeton, J. L. Organometallics 1994, 13, 18511864. (30) Gunnoe, T. B.; White, P. S.; Templeton, J. L. J. Am. Chem. Soc. 1996, 118, 6916-6923.

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approximately 3.2 in dimethyl sulfoxide and is acidic enough to protonate the Ru(II) amide complexes TpRu(CO)(PPh3)(NHPh) (1), TpRu(PMe3)2(NHPh) (2), and TpRu{P(OMe)3}2(NHPh) (3).31 To more accurately quantify the basicity of the three amido complexes TpRu(CO)(PPh3)(NHPh) (1), TpRu(PMe3)2(NHPh) (6), and TpRu{P(OMe)3}2(NHPh) (7), we reacted each with weak acids. Dissolution of TpRu(CO)(PPh3)(NHPh) (1), TpRu(PMe3)2(NHPh) (6), or TpRu{P(OMe)3}2(NHPh) (7) in CD2Cl2 with 1 equiv of methanol (pKa ) 16 in water) results in no observable reaction after 24 h. The 1H NMR spectrum of a combination of TpRu(PMe ) (NHPh) 3 2 (6) with malononitrile in CD2Cl2 reveals an immediate reaction at room temperature. For example, the resonance due to the amido proton of 6 and the methylene protons of NtCCH2CtN are not observed, and all other resonances are shifted relative to starting materials. In addition, broad resonances are observed at approximately 2.6 and 5.4 ppm. Addition of a second equivalent of malononitrile shifts the broad resonance at 2.6 ppm to 3.4 ppm. These results are consistent with the rapid equilibrium shown in eq 4, in which the broad resonances at 2.6 and 5.4 ppm are due to a foursite rapid proton exchange between the amido proton of 6, the amine protons of [TpRu(PMe3)2(NH2Ph)]+, the methylene protons of malononitrile, and the CH proton of the conjugate base of malononitrile (ion paired with the cationic amine complex).

At -90 °C the resonance at 2.6 ppm decoalesces, and new resonances (broad) are observed at 3.8 and 1.6 ppm. The resonances at 3.8 and 1.6 ppm are assigned to the CH2 and CH protons of malononitrile and its conjugate base. Similar (31) Isaacs, N. S. Physical Organic Chemistry; Wiley: New York, 1987.

Influence of the Filled dπ-Manifold and Ligands

results are observed with TpRu(CO)(PPh3)(NHPh) (1) and TpRu{P(OMe)3}2(NHPh) (7). For example, the 1H NMR spectrum of complex 1 with malononitrile -90 °C shows broad resonances between 4.5 and 4.8 ppm (consistent with amido and amine proton chemical shifts), a broad resonance at approximately 3.8 ppm (malononitrile), and a broad resonance at 1.3 ppm (CH of malononitrile conjugate base). These four resonances coalesce at room temperature. The 1H NMR spectrum of a 1/1 mixture of TpRu{P(OMe) } 3 2 (NHPh) (7) and malononitrile shows resonances consistent with a single TpRu complex (presumably the amine and amido complex in rapid equilibrium) and a broad resonance at approximately 2.4 ppm. The resonance at 2.4 ppm is assigned as the time average between the CH2 group of malononitrile and the CH group of the malononitrile conjugate base. Addition of a second equivalent of malononitrile results in a shift of the resonance at 2.4 ppm to approximately 3.4 ppm. To confirm that the amido complexes 6 and 7 are deprotonating malononitrile, UV-vis spectra of THF solutions of the amido complexes with malononitrile were acquired. The use of UV-vis spectroscopy to confirm an acid-base equilibrium of TpRu(CO)(PPh3)(NHPh) (1) and malononitrile was complicated by broad and overlapping absorptions for the amido complex 1 and the conjugate acid amine complex. The amido complex TpRu(PMe3)2(NHPh) (6) exhibits λmax ) 293 nm in its UV-vis spectrum, and the amine complex [TpRu(PMe3)2(NH2Ph)][OTf] absorbs at λmax ) 267 nm. Neither malononitrile nor its lithiated anion absorb at λ > 220 nm. The UV-vis spectrum of a combination of complex 6 with 1 equiv of malononitrile shows an absorption at λmax ≈ 267 nm caused by the formation of the cationic amine (via deprotonation of malononitrile) with a red-shifted shoulder due to the amido complex 6 (λmax ) 293 nm). The combination of amido complex 7 with malononitrile exhibits similar results, and variation of malononitrile concentrations results in the observation of isosbestic points. Due to the π-conflict, it was anticipated that the electronic contribution to the Ru-Namido bond rotation would be mitigated compared with systems in which the amido can efficiently π-donate into an empty metal dπ-orbital. In accord with this, the Ru-Namido rotational barrier for TpRu(CO)(PPh3)(NHPh) (1) was previously determined to be 12 kcal/ mol (at -60 °C),27 and oxidation to Ru(IV) increases the barrier to rotation.32 For complex 1, no evidence of restricted rotation around the Namido-Cphenyl bond is observed down to -100 °C. In contrast, variable-temperature 1H NMR for TpRu(PMe3)2(NHPh) (6) down to -100 °C reveals no evidence for restricted rotation of the Ru-Namido bond. For example, no detectable changes are noted for the Tp, NH, PMe3, or the amido phenyl para proton resonances. At elevated temperatures, single resonances are observed for the ortho and meta protons of the amido phenyl. At reduced

temperatures the ortho and meta resonances broaden and eventually decoalesce (Figure 4). These observations are consistent with hindered rotation around the amido nitrogen and ipso carbon of the phenyl ring, and the barrier at 0 °C has been calculated to be 12.8 kcal/mol. The 1H NMR spectrum of TpRu{P(OMe)3}2(NHPh) (7) at 50 °C reveals single resonances for the phenyl ortho and meta protons. At temperatures between 40 and -45 °C these resonances broaden, decoalesce, and sharpen. In this temperature range there are no observed changes in the appearance of the Tp, NH, P(OMe)3, or phenyl para resonances. However, at -50 °C the ortho and meta phenyl resonances begin to broaden again, and broadening of the resonances for the Tp, P(OMe)3, and phenyl para protons is also observed. The fluxional process at higher temperatures has been assigned as restricted rotation around the Namido-Cphenyl bond. Using the coalescence temperature (-20 °C) for the ortho resonances, a lower limit for the N-C rotational barrier was calculated to be 9.8 kcal/mol. Even though only a lower limit can be reported, no changes in the chemical shifts for the ortho resonances occur between -40 and -50 °C (at which temperature line broadening due to the second process begins). Therefore, the calculated lower limit for the rotational barrier is most likely close in value to the actual barrier. In accord with the observations for TpRu(CO)(PPh3)(NHPh) (1), the second fluxional process has been assigned as hindered rotation about the Ru-N bond; however, at -80 °C the slow exchange region has not been established, and a barrier to rotation cannot be determined. Using Faller’s guide for estimating free energies of activation according to the temperature at which line broadening begins, the Ru-N bond rotational barrier complex 7 is likely to be 7 > 1). The observed trend suggests that increased π-conflict may influence the extent of delocalization of the amido lone pair into the phenyl π*system. In contrast, the expected trend due to steric factors would give a decreasing barrier to rotation in the order 1 > 6 > 7. In addition, we note that the Ru-N rotational barrier is apparently greater for complex 1 than for complexes 6 and 7 (Scheme 3). This is consistent with the possibility of a three-center orbital interaction between the amido ligand, Ru, and CO leading to some multiple bonding between the amido ligand and metal center. In such a scenario, the decreased Ru-N bond rotational barriers for complexes 6 and 7 would be attributed to a combination of increased metal electron density (i.e., a more-donating ligand set) and nearly degenerate orthogonal metal dπ-orbitals. However, the presence of the bulky PPh3 ligand also suggests that steric factors may make a significant contribution to the rotational barrier. As previously reported, the solid-state structure of 1 provides little evidence for a three-center interaction. Therefore, the tentative conclusion is that the increased Ru-N rotational barrier for complex 1 compared with complexes

6 and 7 is due primarily to differences in the steric contributions of the L and L′ ligands, and that π-interactions play a minor role in the barrier to Ru-Namido bond rotation. In contrast, the electronic nature of the metal center seemingly plays an important role in the N-C bond rotational barrier. Summary The basicity of a series of octahedral and d6 phenyl amido complexes with variable ligand sets has been studied, and the pKa’s of the conjugate acid aniline complexes have been estimated to be similar to that of malononitrile. The low acidity of the cationic aniline systems is attributed to the filled dπ-manifold of the metal center. In addition, the impact of the filled dπ-orbitals is observed in the variation of RuNamido and Namido-Cphenyl bond rotational barriers as well as solid-state geometric features. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund (administered by the American Chemical Society) and to North Carolina State University for support of this research. We thank Mr. Roger Sullivan and the Martin research group (NCSU Department of Chemistry) for use of the UV-vis spectrophotometer. Supporting Information Available: Complete tables of crystal data, collection and refinement data, atomic coordinates, bond distances and angles, and anisotropic displacement coefficients for 4, 5, and 7. This material is available free of charge via the Internet at http://pubs.acs.org. IC020163J

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